The present invention relates to a device for fastening a special bolt having a tip which is sheared when subjected to torque exceeding a predetermined magnitude so that the bolt can be fastened by specified torque.
A torque control bolt 9 shown in FIG. 6 has heretofore been used which can be fastened by specified torque to avoid variations in the bolt fastening force and loosening.
The torque control bolt is formed, at the forward end of its threaded shank 94, with a tip 91 to be sheared. When the bolt is fastened with a device designed specifically therefor and subjected to torque exceeding a predetermined magnitude, a stress concentrates on the bottom of a circumferential groove 95 between the threaded shank 94 and the tip 91, and the tip 91 is sheared at the groove 95 by a maximum shearing stress which is dependent on the shape of the groove 95. Accordingly whether the tip 91 is sheared or not apparently indicates whether or not the bolt is completely fastened.
Devices designed specifically for torque control bolts are disclosed, for example, in U.S. Pat. No. 2,928,302, and Japanese Utility Model Application No. 75667/1972 (Published Unexamined Utility Model Application No. 29598/1974) as shown in FIG. 7. Such a disclosed fastening device includes at its forward end an inner socket 6 and an outer socket 7 which are coupled to a drive assembly 1. The inner socket is fitted to the tip 91 to be sheared from the forward end of the bolt shank, and the outer socket is fitted to a nut screwed on the bolt. The drive assembly 1 is operated while causing the bolt tip to withstand the fastening reaction of the nut through the inner socket. The fastening torque of the nut relative to the bolt is controlled by the shearing of the bolt tip. The fastening device described, however, is not provided with means for preventing incomplete fitting of the bolt tip 91 in the inner socket 6, so that the bolt is likely to be fastened, with the bolt tip incompletely fitted in the inner socket because the bolt tip is concealed by the outer socket 7 and unseen from outside.
If the fastening device is operated without fitting the tip fully into the inner socket, i.e. with the tip positioned to receive pressure from the innner socket over a small area, the tip is subjected to an increased surface pressure and becomes unable to withstand the torque, which blunts the ridges of the tip, permitting the tip to rotate idly relative to the socket. This gives rise to the problem that the deformed tip renders the bolt itself no longer reusable, causes damage to the inner surface of the socket and allows the fastening reaction to act inadvertently on the operator by way of the fastening device, hence hazardous.
Further with the fastening device described, the outer socket 7 is fitted to the nut after the inner socket has been fitted to the bolt tip, by supporting the device main body with one hand while pressing the main body toward the bolt, and turning the outer socket 7 with the other hand to fit the socket around the hexagonal nut. This procedure is cumbersome, inefficient and causes fatigue to the operator.
As the nut is screwed on the bolt during the operation of the fastening device, the threaded shank 94 of the bolt progressively moves outward from the surface of the nut. To accommodate the axial movement of the bolt, the fastening device of FIG. 7 includes a holder 3 coupled to the drive assembly 1 and formed with a hexagonal bore 30 as seen in FIG. 8, and the inner socket 6, which is in the form of a hexagonal rod, is slidably fitted in the bore 30, such that as the threaded shank 94 of the bolt 9 moves outward from the nut 92, the inner socket 6 is retracted into the holder 3.
However, when the holder 3 rotates in the direction of an arrow, the corners 68 of the inner socket 6 come into line-to-line contact with the inner surface of the holder 3 since there is a clearance 36 between the inner surface of the holder 3 and the outer surface of the inner socket 6 for permitting sliding.
If torque of increased magnitude acts on the holder 3 which is thus in line-to-line contact with the inner socket 6, the corners 68 of the inner socket 6 will tightly press the holder 3 as if biting in its inner surface, impeding the axial movement of the inner socket, which is therefore unable to retract smoothly. Furthermore, the holder 3, when not having a large wall thickness, fails to withstand the pressure of the corners 68 and is likely to break.
When great frictional resistance acts on the inner socket against its axial movement, the drive assembly 1 will be heavily loaded. The necessity to make the assembly 1 large-sized and the large wall thickness of the holder 3 tend to render the fastening device large-sized in its entirety.